Cassegrain antenna with offset feed



July 18, 1967 5. BROUSSAUD 3,332,083

CASSEGRAIN ANTENNA WITH OFFSET FEED Filed May 25, 1964 4 Sheets-Sheet l Fig.3

July 18, 1967 5. BROUSSAUD 3,332,083

CASSEGRAIN ANTENNA WITH OFFSET FEED Filed May 25, 1964 4 Sheets-Sheet 2 July 18, 1967 G. BROUSSAUD 3,332,083

CASSEGRAIN ANTENNA WITH OFFSET FEED Filed May 25, 1964 4 Sheets-Sheet 3 y 1967 cs. BRoL JssAuD 3,332,083

CASSEGRAIN ANTENNA WITH OFFSET FEED Filed May 25, 1964 4 Sheets-Sheet 4 Fig.6.

United States Patent Ofifice 3,33Zfi83 Patented July 18, 1967 3,332,083 CASSEGRAIN ANTENNA WITH OFFSET FEED Georges Bronssaud, Paris, France, assignor to CSF- Compaguie Generale de Telegraphic Sans Fil, a corporation of France Filed May 25, 1964, Ser. No. 369,752 Claims priority, application France, June 14, 1963, 938,124 Claims. (Cl. 343761) The present invention relates to parabolic antennas i.e. to antennas comprising a parabolic reflector and a source. As is known, this source is generally positioned at or near the paraboloid focus.

One of the essential problems which arise with this general type of antennas is that of obtaining a good illuminating of the reflector by the source while avoiding any diffraction.

In paraboloid antennas there is a spherical wave between the radiator or receiver element placed at the focus of the paraboloid and the reflector.

The coupling between the reflector and the radiator, generally results in diffraction and parasitic radiations.

For example, in a simple parabolic antenna, the radiator is a horn and its mouth is so sized as to insure that energy is concentrated in the spherical wave portion necessary to illuminate the reflector. However, one is thus lead to use horns whose aperture is l to 3 A broad, A being the operating wavelength, which is not enough to avoid parasitic radiations.

It would of course be possible to increase the dimensions of the horn mouth with a view towards better coping with the parasitic radiations. However, this solution would not be very effective for horns with a flare angle of say 50. In addition a substantial part of the reflector would be masked, by the radiator, which is not acceptable for various reasons, well known in the art and affecting the standing wave ratio, the gain, the level of the side lobes etc.

It is an object of the invention to provide a parabolic antenna free of the above disadvantages and wherein the parasitic radiations are low while the directivity is high and the pass band broad.

The invention will be best understood from the following description and appended drawing wherein:

FIG. 1 shows a conventional parabolic antenna with the radiator horn placed at the focus of a paraboloid reflector;

FIG. 2 shows a known parabolic antenna wherein the radiator horn is at the focus of the reflector, but the respective axes of the radiator and the reflector do not coincide;

FIGS. 3 and 4 show horns of a of a known type;

FIG. 5 shows an antenna of the Cassegrainian type;

FIG. 6 shows in section an embodiment of the invention;

FIG. 7 shows in perspective the horn used in the systems of FIG. 6; and

FIG. 8 shows, in section, a still further embodiment of the invention.

The same reference letters are used to designate the same components throughout all the figures.

To avoid the above mentioned drawbacks while still eluding the masking effect, parabolic antennas are sometimes used, wherein the radiator is not at the focus of the reflector or is at the focus, but is not coaxial with the reflector.

FIG. 1 shows a conventional parabolic antenna with a horn CR placed at the focus of the reflector P. In the antenna of FIG. 2 horn CR is still at the focus of the reflector P. However, the respective axes of reflector P and of horn CR make an angle 0, the masking effect being thus avoided.

For a given gain and directivity, the advantages thus obtained increase with increasing 0, but also the antenna overall dimensions.

As far as performance alone is concerned, the best results are obtained, when the horn axis is at right angles with the paraboloid axis, as shown in FIG. 3. It can be readily seen in this figure that the horn aperture surface nearly equals the surface of the paraboloid reflector. The advantages of this antenna are well know: the parasitic radiation and the standing wave ratio are low and the pass-band is broad. However, this is obtained at the costs of the increase of the dimensions, which is due in particular to the fact that the horn flare angle is to be of the order of 20 to 30.

This is still true if the antenna of FIG. 3 is improved as shown in FIG. 4 by guiding the wave onto the reflector. A reflector horn is thus obtained. The lateral walls of horn CR are extended until the paraboloid reflector which they contact, the reflection taking place through aperture 0.

Another system devised for improving the operation of the parabolic antenna is that derived from the Cassegrainian mirror. It is shown in FIG. 5.

The horn of FIG. 1 is replaced by an hyperboloidal reflector H, one focus of which coincides with that of the paraboloid P, which is coaxial with reflector H. Reflector H is illuminated by a horn whose phase center is at the second focus of hyperboloid H. This arrangement has, over that of FIG. 1, the advantage that the radiation horn is more directive and is forwardly directed so that its most important parasitic radiations are located in the vicinity of the axis, i.e. where side lobes level due to the main reflector is itself relatively appreciable (-20 db to -30 db).

Another attractive feature of the Cassegrain system is that the phase distortion of the primary source is greatly reduced.

The antenna according to the invention combines the advantages of the off-set parabolic antenna and of the Cassegrainian antenna.

It incorporates an off-set main par-aboloidal reflector and an hyperbolic reflector, whose focus coincides with that of the parabolic reflector.

FIG. 6 shows in section a horn CR integrally associated with an hyperboloid H, thus forming a reflector horn, shown in perspective in FIG. 7. The reflected waves propagate through aperature O.

The focus F of paraboloid P is also the focus of the hyperboloid. The axes of the paraboloid P and of the hyperboloid H are respectively X and Y The surface of the hyperboloid has to be large enough for the level of its parasitic radiations to be lower than that of the parasitic radiations of paraboloid P. Any masking of the parabolic surface is avoided.

The antenna obtained has a better performance than the Cassegrainian system from the triple point of view of parasitic radiations, standing wave ratio and pass-band, these advantages being obtained at the expense of a small increase in overall dimensions.

According to the invention, the hyperboloid can be positioned so that the axis X thereof is normal to the paraboloid axis Y However, when a receiver antenna equipped with a maser is used, it is desirable to have the latter within easy reach. Therefrom when the antenna is rotatably mounted, the maser is advantageously placed in a fixed position, for example in the vicinity of the rotating joint.

FIG. 8 shows an antenna according to the invention mounted for rotation on a frame 1. The antenna comprises a parabolic reflector 2 and a born 3, having an elbow 50 and an aperture 6 terminated by a hyperboloid 4. The antenna is shown in two different positions. Reflector 2 and horn 6 are mounted on a rotating joint 5, for example for scanning space in azimuth. The antenna is covered by a radome 7 and is supported on a circular track by rollers 9.

By way of example, in the 4000 me. to 6000 me. range, the paraboloid may have a diameter up to 20 meters, the horn dimension crosswise being 1.25 rm. and the hyperbolic rnirror having a 3.25 m. aperture.

Of course, the invention is not limited to the embodiments described and shown which were given solely by way of example.

What is claimed is:

1. An antenna for radiating ultra-high frequency energy comprising: a first reflector having a first focus and a first axis, a second reflector'having a second axis and a second focus coinciding With said first focus, said axis of said reflectors, being substantially inclined to each other,

and a horn for illuminating said second reflector, and

horn having a lateral aperture and a mouth closed by said second reflector.

2. An antenna for radiating ultra-high frequency energy comprising: a first parabolic reflector having a first focus and a first axis,'a second reflector having a second axis and a second focus coinciding with said first focus, said axis of said reflectors being substantially inclined to each other and a horn directed for illuminating said'second reflector, said horn having a lateral aperture and a mouth closed by said second reflector.

3. An antenna for radiating ultra-high frequency energy comprising: a first parabolic reflector having a first focus and a first axis, a second hyperbolic reflector having a second axis and a second focus coinciding with said first focus, said axis ofv said reflectors being substantially inclined to each other and a horn directed for illuminating said second reflector, said horn having a lateral aperture and a mouth closed by said second reflector.

4. An antenna for radiating ultra-high frequency energy comprising: a first parabolic reflector having a first focus and a first axis; an hyperbolic reflector having a second axis and a second focus coinciding with said first focus, said axis of said reflectors being substantially inclined to each other; a horn directed for illuminating .said hyperbolic reflector; said horn having a lateral aperture and a mouth closed by said second reflector; said ture and a mouth closed by said second reflector; said reflectors being solid with each other; and a rotating joint for rotating said antenna and feeding said horn, said horn including an elbow between said joint and said hyperbolic reflector.

References Cited UNITED STATES PATENTS 1 3,243,805 3/1966 Smith 343-781 3,255,455 6/1966 Trentini 343781' FOREIGN PATENTS 577,939 6/1946 Great Britain. 

1. AN ANTENNA FOR RADIATING ULTRA-HIGH FREQUENCY ENERGY COMPRISING: A FIRST REFLECTOR HAVING A FIRST FOCUS AND A FIRST AXIS, A SECOND REFLECTOR HAVING A SECOND AXIS AND A SECOND FOCUS COINCIDING WITH SAID FIRST FOCUS, SAID AXIS OF SAID REFLECTORS, BEING SUBSTANTIALLY INCLINED TO EACH OTHER, 